5.4 GREAT

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5.4.1   Overview of AOTs

GREAT specific instructions and reminders of general issues are given in the following topics below. It is necessary to read the GREAT chapter of the Observer's Handbook before preparing detailed GREAT observations in USPOT. Astronomical Observation Requests (AORs) should be created as described in Chapter 3.

The USPOT Observation drop-down menu lists the four Astronomical Observation Templates (AOTs) available for GREAT: Single Point, Raster Mapping, and OTF Mapping, and OTF Array Mapping. Refer to the Observer's Handbook for a complete description of available combinations of configurations and modes for GREAT.

The USPOT GREAT Main AOR Window contains several frames: Instrument Parameters, Reference Position, and Mapping Parameters (available for Raster Mapping, OTF Mapping, and OTF Array Mapping AORs) with two additional tabs in the OTF Array Mapping AOT (Parameters for Scans and Mapping). Figure 5.4-1 shows an example of the Main AOR Window of a GREAT AOT. The instrument-specific fields are discussed in detail in this chapter. Contact the Help-Desk with any questions.

Figure 5.4-1.
GREAT OTF Array Mapping AOT screenshot

Figure 5.4.1. An example of a GREAT AOT Main AOR Window, using the GREAT OTF Array Mapping AOT

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5.4.2   AOR Fields

Tables 5.4-1, 5.4-2, 5.4-3, and 5.4-4 list the required fields for Phase I and Phase II for each available GREAT AOT. Conditional fields (i.e., fields not editable unless certain parameters are specified) are denoted with a footnote, with a reference to the required field to activate the conditional field. Fields that are not listed in these tables fall under one of three categories: fields not directly editable in USPOT (but may be affected by updating other fields, which are required; for more information on how particular fields may be related, refer to the corresponding sections within the Observer's Handbook—denoted in Tables 5.4-1, 5.4-2, 4.4-3 and 5.4-4 by OH followed by the appropriate section number), fields inteded for use only by SOFIA Support Scientists only, or optional fields.

 
Table 5.4-1.
Single Point Phase I Required Fields for Phase I
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.
Field Location Field Reference
New Target Window Specify Target § 3.4
Instrument Parameters Frame Velocity OH § 6.1.1.1
Mixer 1: Frequency 1 Mixer 2: Frequency 2 1Mixer 2: Frequency 3–5 OH § 6.1.1.2
On + Off Integration Time Time EstimatorOH § 6.2.2
Mixer 2 OH § 6.1.1.2
Main AOR Window Instrument Mode OH § 6.2.1
Mapping Parameters Frame Cycles § 5.4.2.6

Single Point Phase II Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Instrument Parameters Frame Primary Frequency OH § 6.1.1.2
Array Rotation Angle OH § 6.2.3.1
Min Contiguous Exp Time § 5.4.2.5
Target Offset RA Target Offset Dec OH § 6.2.3.1
5Chop Throw § 5.4.2.1
5Chop Angle Coordinate 5Chop Angle OH § 6.2.3.1
Reference Position Frame 2Ref Type 2Reference Name 3RA Offset 3Dec Offset 4RA 4Dec 4Choose Position button OH § 6.2.1
Observing Condition & Acquisition / Tracking Window All fields required. OH § 6.2
 
Table 5.4-2.

Raster Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
New Target Window Specify Target § 3.4
Instrument Parameters Frame Velocity (km/s) OH § 6.1.1.1
Mixer 1: Frequency 1 Mixer 2: Frequency 2 1Mixer 2: Frequency 3–5 OH § 6.1.1.2
Mixer 2
Main AOR Window Instrument Mode OH § 6.2.1
Mapping Parameters Frame Num Steps in the x-direction Num Steps in the y-direction OH § 6.2.1.1
On-source Exposure Time Per Point Time Estimator;      OH§ 6.2.2 
Cycles § 5.4.2.6

Raster Phase II Required Fields for Phase II
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Instrument Parameters Frame Primary Frequency OH § 6.1.1.2
Array Rotation Angle
Main AOR Window 5Chop Throw 5Chop Angle Coordinate 5Chop Angle § 5.4.2.1; OH § 6.2.1; OH § 6.2.3.1
Reference Position Frame 2Ref Type 2Reference Name 3RA Offset 3Dec Offset 4RA 4Dec 4Choose Position button OH § 6.2.1
Mapping Parameters Frame Map Offset RA Map Offset Dec MapAngle Step Size in the x-direction Step Size in the y-direction   OH § 6.2.3.1   OH § 6.2.1.1
Number of map points per load OH § 6.2.1.1
Min Contiguous Exp Time § 5.4.2.5
Observing Condition & Acquisition / Tracking Window All fields required. OH § 6.2
 
Table 5.4-3.

OTF Mapping Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
New Target Window Specify Target § 3.4
Instrument Parameters Frame Velocity (km/s) OH § 6.1.1.1
Mixer 1: Frequency 1 Mixer 2: Frequency 2 1Mixer 2: Frequency 3–5 OH § 6.1.1.2
Mixer 2
Main AOR Window Instrument Mode OH § 6.2.1
Mapping Parameters Frame 2On-source Exposure Time Per Point Cycles § 5.4.2.5
Num Steps in the x-direction Num Steps in the y-direction OH § 6.2.3.1

OTF Mapping Phase II Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Instrument Parameters Frame Primary Frequency Array Rotation Angle OH § 6.1.1.2
Main AOR Window 5Chop Throw 5Chop Angle Coordinate 5Chop Angle OH § 6.2.1; OH § 6.2.3.1
Reference Position Frame 2Ref Type 3RA Offset 3Dec Offset 4Choose Position button § 5.4.2.1; OH § 6.2.3
Mapping Parameters Frame Min Contiguous Exp Time § 5.4.2.5
Map Offset RA Map Offset Dec ScanDirection ScanDirectionVector ScanOrder MapAngle Step Size in the x-direction Step Size in the y-direction OH § 6.2.3.1
Observing Condition & Acquisition / Tracking Window All fields required. OH § 6.2
Table 5.4-4.

OTF Array Mapping Phase I Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
New Target Window Specify Target § 3.4
Instrument Parameters Frame Velocity OH § 6.1.1.1
Mixer 1: Frequency 1 Mixer 2: Frequency 2 1Mixer 2: Frequency 3–5 OH § 6.1.1.2
  Mixer 2  OH § 6.1.1.2
  Primary Frequency OH § 6.1.1.2
Main AOR Window Instrument Mode OH § 6.2.1
Mapping Tab Cycles Step size along OTF line § 5.4.2.6; OH § 6.2.3.3
Parameters for Scans Tab 6All Fields Required. OH § 6.2.3.3

OTF Array Mapping Phase II Required Fields
All Observer's Handbook (OH) Reference links in the table below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Location Field Reference
Instrument Parameters Frame Array Rotation Angle OH § 6.2.3.1
Main AOR Window 5Chop Throw 5Chop Angle 5Chop Angle Coordinate § 5.4.2.1; OH § 6.2.1; OH § 6.2.3.1
Mapping tab Number of OTF lines within one block Scan Lines Per Off ScanDirection § 5.4.2.6OH § 6.2.3.3
Reference Position Frame 2Ref Type 3RA Offset 3Dec Offset 4Choose Position button § 5.4.2.1; OH§ 6.2.3
Main AOR Window, Parameters for Scans Tab Min Contiguous Exp Time § 5.4.2.5
MapAngle Map Offset RA Map Offset Dec 6ScanDirectionVectorX 6ScanDirectionVectorY 6ScanOrderX 6ScanOrderY OH § 6.2.3.3
Observing Condition & Acquisition / Tracking Window All fields required. OH § 6.2
1For Mixer 2 = GRE_4G
2For Instrument Mode = Total Power
3For Instrument Mode = Total Power and Ref Type = By Offset
4For Instrument Mode = Total Power and Ref Type = By Position
5For Instrument Mode = Dual Beam Switch or Single Beam Switch
6For ScanDirection = x direction, all fields in the Parameters for scans in x direction Frame are required. For ScanDirection = y direction, all fields in the Parameters for scans in y direction Frame are required. For ScanDirection = x and y directions, all fields in the Parameters for scans in x direction Frame and Parameters for scans in y direction Frame are required. ScanDirection is located in the Parameters for Scans tab.
Return to:   Table 5.4-1   |   Table 5.4-2   |   Table 5.4-3   |   Table 5.4-4  
 

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5.4.2.1   Instrument Mode

Depending on the AOT selected, the Instrument Mode field offers the options of Total Power, Single Beam Switch, and Dual Beam Switch.

In Single Beam Switch or Dual Beam Switch mode, a Chop Angle and Chop Throw must be specified in their respective fields. The Chop Angle is defined in the North through East direction. The Chop Throw refers to the angular separation between the source and reference positions. In Dual Beam Switch mode, there are two reference positions that are determined by (1) the Chop Throw and the Chop Angle and (2) the Chop Throw and the Chop Angle plus 180 degrees.

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5.4.2.2   Rest Frequency

After designating the specific mixers to use in the Mixer 1 and Mixer 2 fields, enter the rest frequencies of the line to be observed. For HFA-LFA configuration, Frequency 1 and Frequency 2 should be entered. For the HFA-4GREAT configuration, Frequencies 1 to 5 should be entered.

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5.4.2.3   Source Velocity

In the Velocity field, enter the source velocity in the local standard of rest frame (VLSR). With this information, the flight team will decide the optimal local oscillator (LO) frequencies for observing these lines (upper side band (USB) versus lower side band (LSB), position of the line in the intermediate frequency (IF)), taking into account frequency-dependent system performance and atmospheric transmission. Specific requests, such as observing other lines in the same setting, must be noted in the comments section of the AOR (accessible via the Comments... button at the bottom of the AOR editing window).

Users searching for a line or uncertain about the line's strength or velocity should do at least one frequency shift. The easiest way to do this is to create a second AOR and to change the source velocity VLSR by 10–20 km/s. The mixer will have to be retuned and a new calibration done, which involves a small cost in time (~2 minutes).

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5.4.2.4   Integration Time & Min Contiguious Exposure Time

For Single Point observations, the ON + OFF Integration Time per point on the sky must be entered. This should not be more than ~60 seconds in Total Power Mode (40 seconds for the HFA) or ~80 seconds  for Single or Dual Beam Switch Mode. If longer exposures are needed, which is almost always the case, please increase the number of cycles.

For all other AOTs, the integration time per point on the sky must be entered in the On-source Exp. Time Per Point field. The OFF integration time will be calculated automatically depending on the observing mode and how the observations are set up. For maps, the Cycles field indicates the number of times the maps are repeated.

Plan each AOR so that one cycle does not take longer than about half an hour to execute. Instead of making a big OTF map, make it into a set of smaller maps. With the Min Contiguous Exposure Time field, users can specify the minimum acceptable observing time in case the observation has to be split over multiple parts for, e.g., efficient flight planning.

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5.4.2.5   Reference Position Frame

For Total Power observations, choose the Reference Position to be as close to the source as possible. It should be no farther away than half a degree and preferably within 10–15 arcmin of the source. A Reference Position more than half a degree away from the target is not likely to work and may result in poor baselines.

For beam-switched observations, a Chop Throw of 100 arcsec or less should be sufficient. Large chop throws affect the pointing of the telescope, introduce coma into the beam, and degrade the data quality.

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5.4.2.6   Other OTF Common Mapping Parameters

USPOT refers to scan rows as along x-direction and column as along y-direction. If the map rotation angle is not zero (the default), however, the map is rotated with respect to the RA/Dec frame. If in doubt about the map orientation, use the Overlays to Current AOR option to plot the OTF coverage on a sky map.

Common Mapping Parameters
All Observer's Handbook (OH) links below point to the latest version of the Observer's Handbook—currently Cycle 8. Be sure you are using the version of the Observer's Handbook that corresponds to your observing cycle. The documentation for all cycles can be found on the Proposal Documents webpage.

Field Specific Instructions
MapAngle (deg) The angle of the x/y map coordinate system relative to the RA/Dec coordinate system. MapAngle is measured in the counter-clockwise direction. The map is rotated around the map center, as defined by the target and map offsets. The allowed values for MapAngle range from 0° to 360°.
Map Offset RA(arcsec) The offset in Right Ascension of the map center from the specified target position.
Map Offset Dec (arcsec) The offset in Declination of the map center from the specified target position
Number of OTF lines within one block The number of scans that make up a single block. A value of 1 will result in an under-sampled map, a value of 2 is the minimum required for a fully sampled map, and a value greater than 2 will result in an over-sampled map (see Section 6.2.3.3 of the Observer's Handbook).
Step size along OTF line The spacing between individual spectra (dumps) along the scan row (in arcseconds). The default value of 6 arcsec results in fully sampled maps.
Scan Lines Per Off The number of map scans performed for each observation of the reference position. If the duration of a single scan is close to 30 seconds, this number should be 1. If the duration of a single scan is very short (e.g., 10 seconds), this can be increased to 2. Note, however, that longer scan durations result in higher on-source observing efficiency.
ScanDirection The scan direction(s) that will be used in the map. The selection here (x and y directions, x direction, y direction) will determine the available fields in the Parameters for scans in x/y direction sections of the AOT.
Exposure Time per Cycle (sec) The time, including overhead, to complete one repetition of the map. This is calculated automatically.
Cycles The number of repetitions desired for the map.

Number of blocks in scan direction

For the Left example in Figure 6-7 of the Observer's Handbook, this is 3.
Number of blocks perpendicular to scan direction For the Left example in Figure 6-7 of the Observer's Handbook, this is 2.
On-source Exp. Time per Point (sec) The amount of on-source integration time for each dump, in each scan. This value is typically between 0.3 and 2.0 seconds.
Scanning Duration The maximum scanning duration for each observation of the reference position is 30 seconds. If the maximum duration exceeds 30 seconds, then one or more of the parameters in the above equation needs to be adjusted.

 

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